1. Field of the Invention
The present invention generally relates to an apparatus for dropping balls into a wellbore. More particularly, the invention relates to a sub, such as a cement plug, capable of selectively releasing balls and other objects into a wellbore, such as during cementing operations. The invention further relates to a pressure equalizer and cross-over device as might be employed during a fluid circulation operation.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or “hung” off of the existing casing. Afterwards, the second casing string is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In the process of forming a wellbore, it is sometimes desirable to utilize various plugs. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. Plugs are commonly used, for example, during the cementing operation for a liner. Plugs are also used during subsea operations for cementing casing.
The process of cementing a liner or other casing string into a wellbore typically involves the use of two different types of plugs—liner wiper plugs and drill-pipe darts. The first plug used is a liner wiper plug. The liner wiper plug is typically run into the wellbore with the liner at the bottom of a working string. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner.
The liner wiper plug has a cylindrical bore formed therein to receive fluids as the liner is lowered into the wellbore. After a sufficient volume of circulating fluid or cement has been placed into the wellbore, a first drill pipe dart or pump-down plug, is deployed. Using drilling mud, cement, or other displacement fluid, the dart is pumped into the working string. As the dart travels downhole, it seats against the liner wiper plug, closing off the internal bore through the liner wiper plug. Hydraulic pressure above the dart forces the dart and the wiper plug to dislodge from the bottom of the working string and to be pumped down the liner together. This forces the circulating fluid or cement that is ahead of the wiper plug and dart to travel down the liner and out into the liner annulus.
In many fluid circulation operations, it is desirable to employ a multi-plug system. In a multi-plug system, two or more plugs are stacked one on top of the other. Each plug has a hollow mandrel defining a fluid passageway therein. Further, each hollow mandrel includes a seat for receiving a separate dart or ball. A first “bottom” plug is launched by dropping a first dart down the working string until it lands in the bottom seat. Fluid is injected into the working string under additional pressure, causing the bottom plug, with the dart landed therein, to be separated from the top plug or plugs. Typically, separation is accomplished by applying sufficient pressure to overcome a shearable connection along the mandrel, and a collet connection.
The bottom wiper plug and dart are commonly used to separate a column of wellbore fluid from a column of cement. The bottom wiper plug and dart are pumped downhole ahead of the cement slurry. The bottom wiper plug and dart exit the working string and travel down the liner. Ultimately, the bottom wiper plug and dart land in a float collar disposed proximate to the bottom of the liner. Pressure is again raised within the wellbore, causing a disk within the bottom plug to burst. Cement is then allowed to flow through the bottom plug and up the annulus outside of the liner.
After a sufficient volume of cement has been injected into the wellbore, a second dart is dropped from the surface. The second dart lands in the fluid passageway of the second (usually, the top) wiper plug. This again effectuates a substantial seal of fluid within the wellbore. Fluid continues to be injected into the wellbore, raising the pressure against the top plug. A shearable connection between the top plug and the mandrel is sheared, allowing the top plug and top dart to be pumped downhole, thereby pushing cement down the liner and then back up the annulus.
Certain limitations and disadvantages exist with the use of cement wiper plugs. The first limitation relates to the restricted size of the mandrel within the plugs. Those of ordinary skill in the art will appreciate that the mandrel in the bottom plug must be smaller than the mandrel in the top plug. This is necessary in order to allow the bottom dart to pass through the seat in the top plug so as to release the bottom plug without releasing the top plug. The restricted bore diameter in the mandrel of the bottom plug serves as a limitation to the rate at which fluid can be pumped downhole. It further serves as a limitation as to the size of balls that can be dropped through the wiper plugs in order to actuate tools further downhole, e.g., an auto-fill float collar disposed near the bottom of the liner. Of course, other tools deployed in the wellbore during a cementing operation will also have a limited diameter available. Thus, one problem frequently encountered in many wellbore operations is the need to overcome the limitation of a restriction in the wellbore that prevents the use of a ball below that restriction. In other words, a ball having a greater diameter than the bore of a tool cannot be dropped through that tool. Typically, a ball having a maximum diameter of 2.25 inches can be used.
For purposes of the present application, the term “ball” includes any spherical or other object, e.g. bars, and plugs, that are dropped into a wellbore. Typically a ball is used downhole to activate a tool or to temporarily seal the wellbore.
A present application pending before the United States Patent and Trademark Office addresses a system that permits a larger-diameter ball to be dropped from below the point of a wellbore restriction. That application is US 2001/0045288, published Nov. 29, 2001. The listed inventor is Allamon. In one embodiment, shown in FIGS. 8 and 9 therein, a sub is attached to the bottom of a cement plug. The sub includes a large-ball seat for receiving a larger-diameter ball. The sub also includes a smaller seat for receiving a smaller, releasing ball. Further, the sub includes a sleeve that moves downward in response to pressure after the smaller ball has been dropped and seated, thereby closing off flow-through ports. The larger-diameter ball is released through the injection of fluid under pressure after the smaller, releasing ball is dropped and after the flow-through ports are closed. The seats are fabricated from a yieldable material such as aluminum that permits the balls to drop at a predetermined level of fluid pressure.
The above pending application has utility in the dropping of a ball that would otherwise be of a diameter that is too large to pass through the restrictions above the liner wiper plug. However, the described system requires refabrication of the liner wiper plug to accommodate an integral ball releasing apparatus, to wit, a frangible seat within the plug. It further requires fabrication of ports in the plug above the seat for the larger ball.
Another disadvantage to the use of a dual or multi-plug system relates to the potential for excessive pressure building up on the outside of the top plug after the bottom plug has been launched. This condition may arise in a variety of circumstances. For example, if a portion of formation collapses around the liner prior to or during a cementing operation, it is necessary to raise the level of circulation pressure in order to circulate out the bridged formation. In this instance, circulation fluid will exit relief ports within the working string and act downwardly against the top plug from outside of the working string. This creates the potential for premature launch of the top plug.
The presence of unwanted pressure on the outside of the top wiper plug may also arise during the setting of an auto-fill float collar. Unwanted pressure buildup could also occur while actuating a hydraulically set liner hanger, or during a staged cementing operation.
To overcome the problem of excessive pressure acting against the top plug from outside of the working string, some drilling operators utilize a pressure equalizer tool. A pressure equalizer tool is typically installed in the working string above the cement plug and below the running tool. The pressure equalizer allows fluid to be received back into the working string from above the cement plug where a positive pressure differential is sensed. However, this requires the deployment of a separate tool on top of the cement plugs.
Therefore, there is a need for a more effective plug-dropping apparatus for a cementing plug. There is a further need for a cementing plug having a mechanism for suspending and selectively releasing a ball, thereby overcoming wellbore restrictions within and above the cement plug. Still further, there is a need for a ball-releasing mechanism that can be easily installed into a conventional cement plug. Further still, there is a need for a cement plug having an integral pressure equalizer/cross-over tool.